Biomimetic nanoparticles are known to serve as nanoscale adjuvants, enzyme mimics and amyloid fib... more Biomimetic nanoparticles are known to serve as nanoscale adjuvants, enzyme mimics and amyloid fibrillation inhibitors. Their further development requires better understanding of their interactions with proteins. The abundant knowledge about proteinprotein interactions can serve as a guide for designing protein-nanoparticle assemblies, but the chemical and biological inputs used in computational packages for protein-protein interactions are not applicable to inorganic nanoparticles. Analysing chemical, geometrical and graph-theoretical descriptors for protein complexes, we found that geometrical and graph-theoretical descriptors are uniformly applicable to biological and inorganic nanostructures and can predict interaction sites in protein pairs with accuracy >80% and classification probability ~90%. We extended the machine-learning algorithms trained on proteinprotein interactions to inorganic nanoparticles and found a nearly exact match between experimental and predicted interaction sites with proteins. These findings can be extended to other organic and inorganic nanoparticles to predict their assemblies with biomolecules and other chemical structures forming lock-and-key complexes.
Twisted graphene quantum dots, with chiral amino acids around their edges, enable a new biocompat... more Twisted graphene quantum dots, with chiral amino acids around their edges, enable a new biocompatible nanomaterial that displays chiral selectivity when interacting with live cells.
The emphasis in this paper is on presenting a new methodology, together with some illustrative ap... more The emphasis in this paper is on presenting a new methodology, together with some illustrative applications, for the study of polycyclic aromatic hydrocarbon polymerization leading to soot, widely recognized as a very important and challenging combustion problem. The new code, named fully integrated Kinetic Monte Carlo/Molecular Dynamics (KMC/MD), places the two simulation procedures on an equal footing and involves alternating between KMC and MD steps during the simulation. The KMC/MD simulations are used in conjunction with high-level quantum chemical calculations. With traditional kinetic rates and dealing with the growth of particles, it is often necessary to perform a lumping procedure in which much atomic-scale information is lost. Our KMC/MD approach is designed to preserve atomic-scale structure: a single particle evolves in time with real three-dimensional structure (bonds, bond angles, dihedral angles). In this paper, the methodology is illustrated by a sample simulation of high molecular mass compound growth in an environment (T, H, H 2 , naphthalene, and acenaphthylene concentrations) of a low-pressure laminar premixed benzene/oxygen/argon flame with an equivalence ratio of 1.8. The use of this approach enables the investigation of the physical (e.g., porosity, density, sphericity) as well as chemical (e.g., H/C, aromatic moieties, number of cross-links) properties.
An application of the Reaction Class Transition State Theory/Linear Energy Relationship (RC-TST/L... more An application of the Reaction Class Transition State Theory/Linear Energy Relationship (RC-TST/LER) is presented for the evaluation of the thermal rate constants of hydrogen abstraction reactions by H atoms from Polycyclic Aromatic Hydrocarbons (PAH). Two classes of reactions have been considered, namely hydrogen bonded to six-and five-membered rings, respectively, and twenty-two reactions have been used to develop the RC-TST/LER parameters. B3LYP and BH&HLYP density functional theory methods were used to calculate necessary potential energy surface information. Detailed analyses of RC-TST/LER reaction factors lead to the conclusion that rate constants for any reaction in these two classes can be approximated by those of its corresponding principal reaction corrected by the reaction symmetry factor. Specifically, for hydrogen abstraction from six-membered rings such as naphthalene and pyrene, k(T)) (σ/σ H+C 6 H 6) k H+C 6 H 6) (σ/6)-{1.42 × 10 8 T 1.77 exp(-6570/T)}(cm 3 /mol‚s), and for hydrogen abstraction from five-membered rings such as acenaphthylene and acephenanthrylene, k(T)) (σ/σ H+C 12 H 8) k H+C 12 H 8) (σ/2){3.27 × 10 8 T 1.71 exp(-8170/T)} (cm 3 /mol‚s), where σ is the reaction symmetry number.
Reaction pathways for polyaromatic growth in combustion environments are explored theoretically u... more Reaction pathways for polyaromatic growth in combustion environments are explored theoretically using molecular dynamic techniques. The reaction systems considered include cyclization reactions that involve H atom migration. Specifically we examine the absorption of acetylene on an aromatic ring radical. The Parallel Replica method for accelerating Molecular Dynamics calculations is used to screen reaction pathways. The information obtained through the use of Molecular Dynamics runs are then analyzed with quantum chemical methods and thermodynamic data for the key compounds.
The processes involved in soot precursor formation exhibit a wide range of timescales, spanning p... more The processes involved in soot precursor formation exhibit a wide range of timescales, spanning pico- or nanoseconds for intramolecular processes that can occur on a particle surface to milliseconds for the formation of the first soot precursors. To accurately describe the soot formation process, it is important to model the reactions happening at different timescales. The use of atomistic models
... A.VIOLI, S. YAN, EG EDDINGS*, AND AF SAROFIM ... Jet A and JP-8 are kerosene fuels used in av... more ... A.VIOLI, S. YAN, EG EDDINGS*, AND AF SAROFIM ... Jet A and JP-8 are kerosene fuels used in aviation and consist of complex mixtures of higher order hydrocarbons, including alkanes, cycloalkanes, and aromatic molecules. ...
Formation pathways for high-molecular-mass compound growth are presented, showing why reactions b... more Formation pathways for high-molecular-mass compound growth are presented, showing why reactions between aromatic moieties are needed to explain recent experimental findings. These reactions are then analyzed by using quantum mechanical density functional methods. A sequence of chemical reactions between aromatic compounds (e.g., phenyl) and compounds containing conjugated double bonds (e.g., acenaphthylene) was studied in detail. The sequence begins with the H-abstraction from acenaphthylene to produce the corresponding radical, which then furnishes higher aromatics through either a two-step radical-molecule reaction or a direct radical-radical addition to another aromatic radical. Iteration of this mechanism followed by rearrangement of the carbon framework ultimately leads to high-molecular-mass compounds. This sequence can be repeated for the formation of high-molecular-mass compounds. The distinguishing features of the proposed model lie in the chemical specificity of the routes considered. The aromatic radical attacks the double bond of five-membered-ring polycyclic aromatic hydrocarbons. This involves specific compounds that are exceptional soot precursors as they form resonantly stabilized radical intermediates, relieving part of the large strain in the five-membered rings by formation of linear aggregates.
Purpose-This paper aims to study the natural convection of a nanofluid inside a cavity which cont... more Purpose-This paper aims to study the natural convection of a nanofluid inside a cavity which contains obstacles using lattice Boltzmann method (LBM). The results have focused mainly on various parameters such as number and aspect ratio of roughness elements and different nanoparticle volume fraction. The isotherms and streamlines are presented to describe the hydrodynamics and thermal behaviors of the nanofluid flow throughout the enclosure. Design/methodology/approach-The methodology of this paper consists of mathematical model, statement of the problem, nanofluid thermophysical properties, lattice Boltzmann method, LBM for fluid flow, LBM for heat transfer, numerical strategy, boundary conditions, Nusselt (Nu) number calculation, code validation and grid independence. Findings-Natural convection heat transfers of a nanofluid inside cavities with and without rough elements have been studied. Lattice Boltzmann technique has been used as numerical approach. The results showed that at higher Rayleigh number (Ra = 10 6), there are denser streamlines near the left (source) and right wall (sink) which results in better cooling and enhances convective heat rejection to the heat sink. After a distinctive aspect ratio of rough elements (A = 0.1), change in streamline pattern which arises from increasing of aspect ratio does not have an important effect on isotherms. Results indicate that for lower Rayleigh number (Ra = 10 3), no variation in average Nu is observed with increasing in number of roughness, while for higher one (Ra = 10 6) average Nu decreases from N = 0 (smooth cavity) up to N = 4 and then remains constant (N = 6). Originality/value-Currently, no argumentative and comprehensive extraction can be concluded without fully understanding the role of different arrangement of roughness. Some geometrical parameters such as aspect ratio, number and position of rough elements have been considered. Also, the effect of nanoparticle concentration was studied at different Ra number. Briefly, using LBM, this paper aims to investigate the natural convection of a nanofluid flow on the thermal and hydrodynamics parameters in the presence of rough element with various arrangements.
Homogeneous nucleation is identified as one of the steps responsible for the formation of the cri... more Homogeneous nucleation is identified as one of the steps responsible for the formation of the critical nuclei that define the transition from gas-phase species to soot formation particles. However, the effect of molecular properties (like size, composition, and shape) on their propensity to physically bind, for a time long enough to initiate particle nucleation, has not been thoroughly investigated. In this work, we analyze the effect of oxygen content on the homodimerization propensity of different polycyclic aromatic compounds (PACs) using molecular dynamics simulations coupled with well-tempered Metadynamics enhanced sampling. We analyzed species in two mass groups, with a molecular weight ranging from 448 μ to 696 μ, and with varying oxygen content, mostly present as ethers. The reconstructed free energy profiles show that, besides mass, the presence of oxygenated groups influences the tendency of PACs to bind, with oxygenated compounds forming dimers that are less stable than the corresponding polycyclic aromatic hydrocarbons. Although this work represents only the first step towards a model for PACs dimerization that include oxygen, these results suggest that oxygenated compounds are less likely to condense onto soot particles.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
The emission standards of combustion have been steadily reduced in recent years, and a large rese... more The emission standards of combustion have been steadily reduced in recent years, and a large research effort has been focused on lowering the emissions of hydrocarbons and particulate matter. Addition of oxygenates to fuel reduces these pollutants. In this paper, we report on a detailed investigation of the growth mechanisms for gas-phase species in ethylene/air and ethylene/ethanol/air flames in order to assess the importance of various chemical mechanisms for the molecular growth of soot precursors. We employ a variety of computational techniques that include stochastic and deterministic methods to study the formation of gas-phase species and their growth into soot precursors via chemical and physical mechanisms. We have drawn the following conclusions: 1) the chemistry of oxygenated compounds (specifically the high concentrations of O 2 and OH) is critical to reproduce the experiments; 2) using free energy simulations, we found out that the tendency of molecules to form dimers is mainly affected by the molecular shape rather than the mass of the aggregate. Finally we propose a different mechanism for the growth of soot precursors based on radicalradical recombination to form molecules of high molecular masses (> 1200 u). These structures are then likely to promote physical aggregation to further the growth mechanism.
... IN HIGH-PRESSURE DIESEL-LIKE CONDITIONS ANGELA VIOLI,1 ANDREA D ANNA,1 ANTONIO D ALESSIO,1 MA... more ... IN HIGH-PRESSURE DIESEL-LIKE CONDITIONS ANGELA VIOLI,1 ANDREA D ANNA,1 ANTONIO D ALESSIO,1 MASSIMO ASTARITA2 and BIANCA MARIA VAGLIECO2 1Dipartimento di ... 12. Astarita, M., Corcione, FE, Vaglieco BM, and Val entino, G., SAE paper 99-0510. 13. ...
In this paper, we report a detailed analysis of the breakdown kinetic mechanism for methyl butano... more In this paper, we report a detailed analysis of the breakdown kinetic mechanism for methyl butanoate (MB) using theoretical approaches. Electronic structures and structure-related molecular properties of reactants, intermediates, products, and transition states were explored at the BH&HLYP/cc-pVTZ level of theory. Rate constants for the unimolecular and bimolecular reactions in the temperature range of 300-2500 K were calculated using Rice-Ramsperger-Kassel-Marcus and transition state theories, respectively. Thirteen pathways were identified leading to the formation of small compounds such as CH 3 , C 2 H 3 , CO, CO 2 , and H 2 CO. For the initial formation of MB radicals, H, CH 3 , and OH were considered as reactive radicals participating in hydrogen abstraction reactions. Kinetic simulation results for a high temperature pyrolysis environment show that MB radicals are mainly produced through hydrogen abstraction reactions by H atoms. In addition, the C(O)OCH 3) CO + CH 3 O reaction is found to be the main source of CO formation. The newly computed kinetic sub-model for MB breakdown is recommended as a core component to study the combustion of oxygenated species.
A detailed kinetic model has been developed and used to simulate aromatic growth in premixed benz... more A detailed kinetic model has been developed and used to simulate aromatic growth in premixed benzene and ethylene flames. The model considers the role of resonantly stabilized radicals in the growth of aromatic species, in addition to the hydrogen abstraction carbon addition (HACA) mechanism, which involves hydrogen abstraction to activate aromatics followed by subsequent acetylene addition. Model results show that the self-combination of resonantly stabilized radicalss in particular, the combination of cyclopentadienyl radicalssis the controlling pathway for the aromatic-ring growth. The kinetic model reproduces the experimental trends of these compounds already below the flame front and their decrease within the flame front, which has been observed experimentally but never predicted numerically. The reaction mechanism has been used to identify the different behaviors of aromatic growth in ethylene and benzene flames. Benzene formation is the rate-limiting step for aromatic growth in ethylene flames. C 6 H 6 is formed across the flame front and carbon growth continues through the formation of multi-ring aromatics in the postflame region. In benzene oxidation, the aromatic ring is already present in the main oxidation zone and it is mainly oxidized to cyclopentadienyl radicals. As a consequence, a large amount of cyclopentadienyl radicals are available for recombination reactions, leading to multi-ring formation already in the main oxidation region. The increase of the temperature at the flame front reduces their concentration due to pyrolysis and/or oxidation resulting in a peak value in the main oxidation region and a leveling-off in the post-oxidation region of the flame.
Classical molecular dynamics simulations of atomistic models of combustion-generated carbon nanop... more Classical molecular dynamics simulations of atomistic models of combustion-generated carbon nanoparticles and lipid bilayers have been performed to explore their possible structural, dynamical, and thermodynamic effects on biological membranes. The DREIDING generic force field is used for the carbonaceous nanoparticles of different morphologies, as produced from combustion sources, and the united atom model was employed for the dimyristoylphosphatidylcholine (DMPC) bilayer. It is observed that particle shape and structure have significant effects on solvation, mobility, adsorption, and permeation behavior of the particles. While combustion-generated carbon nanoparticles with an aspect ratio close to unity prefer to stay near the membrane center, precursors with other shapes mostly reside within the hydrocarbon tail region of the membrane. Carbon nanoparticles are not trapped in a local region even inside the membranes but move freely with a speed depending on their molecular weight. The adsorption of the particles on the surface of the biological membrane is comparable to thermal fluctuations because the weak segregation effect by water molecules is the main driving force to the adsorption behavior. The bigger the precursors are, the stronger they are bound to the membrane surface. The presence of combustion-generated nanoparticles inside the membrane perturbs local lipid density by pushing the neighboring lipid molecules away from the nanoparticles. This, coupled with thermal fluctuations, can induce an instantaneous membrane pore to allow water protrusion. From the umbrella sampling method, the potential of mean force for the permeation of carbona nanoparticles into the bilayer was also obtained. Surprisingly, elongated particles have a free energy barrier an order of magnitude smaller compared with more round ones. In addition, the round carbon nanoparticles showed strong hysteresis due to the local trapping of water molecules. Although the carbon soot precursors studied in this work are not the well-known carbon nanoparticles such as fullerenes or carbon nanotubes, the qualitative features of this study may be applicable to them as well.
Biomimetic nanoparticles are known to serve as nanoscale adjuvants, enzyme mimics and amyloid fib... more Biomimetic nanoparticles are known to serve as nanoscale adjuvants, enzyme mimics and amyloid fibrillation inhibitors. Their further development requires better understanding of their interactions with proteins. The abundant knowledge about proteinprotein interactions can serve as a guide for designing protein-nanoparticle assemblies, but the chemical and biological inputs used in computational packages for protein-protein interactions are not applicable to inorganic nanoparticles. Analysing chemical, geometrical and graph-theoretical descriptors for protein complexes, we found that geometrical and graph-theoretical descriptors are uniformly applicable to biological and inorganic nanostructures and can predict interaction sites in protein pairs with accuracy >80% and classification probability ~90%. We extended the machine-learning algorithms trained on proteinprotein interactions to inorganic nanoparticles and found a nearly exact match between experimental and predicted interaction sites with proteins. These findings can be extended to other organic and inorganic nanoparticles to predict their assemblies with biomolecules and other chemical structures forming lock-and-key complexes.
Twisted graphene quantum dots, with chiral amino acids around their edges, enable a new biocompat... more Twisted graphene quantum dots, with chiral amino acids around their edges, enable a new biocompatible nanomaterial that displays chiral selectivity when interacting with live cells.
The emphasis in this paper is on presenting a new methodology, together with some illustrative ap... more The emphasis in this paper is on presenting a new methodology, together with some illustrative applications, for the study of polycyclic aromatic hydrocarbon polymerization leading to soot, widely recognized as a very important and challenging combustion problem. The new code, named fully integrated Kinetic Monte Carlo/Molecular Dynamics (KMC/MD), places the two simulation procedures on an equal footing and involves alternating between KMC and MD steps during the simulation. The KMC/MD simulations are used in conjunction with high-level quantum chemical calculations. With traditional kinetic rates and dealing with the growth of particles, it is often necessary to perform a lumping procedure in which much atomic-scale information is lost. Our KMC/MD approach is designed to preserve atomic-scale structure: a single particle evolves in time with real three-dimensional structure (bonds, bond angles, dihedral angles). In this paper, the methodology is illustrated by a sample simulation of high molecular mass compound growth in an environment (T, H, H 2 , naphthalene, and acenaphthylene concentrations) of a low-pressure laminar premixed benzene/oxygen/argon flame with an equivalence ratio of 1.8. The use of this approach enables the investigation of the physical (e.g., porosity, density, sphericity) as well as chemical (e.g., H/C, aromatic moieties, number of cross-links) properties.
An application of the Reaction Class Transition State Theory/Linear Energy Relationship (RC-TST/L... more An application of the Reaction Class Transition State Theory/Linear Energy Relationship (RC-TST/LER) is presented for the evaluation of the thermal rate constants of hydrogen abstraction reactions by H atoms from Polycyclic Aromatic Hydrocarbons (PAH). Two classes of reactions have been considered, namely hydrogen bonded to six-and five-membered rings, respectively, and twenty-two reactions have been used to develop the RC-TST/LER parameters. B3LYP and BH&HLYP density functional theory methods were used to calculate necessary potential energy surface information. Detailed analyses of RC-TST/LER reaction factors lead to the conclusion that rate constants for any reaction in these two classes can be approximated by those of its corresponding principal reaction corrected by the reaction symmetry factor. Specifically, for hydrogen abstraction from six-membered rings such as naphthalene and pyrene, k(T)) (σ/σ H+C 6 H 6) k H+C 6 H 6) (σ/6)-{1.42 × 10 8 T 1.77 exp(-6570/T)}(cm 3 /mol‚s), and for hydrogen abstraction from five-membered rings such as acenaphthylene and acephenanthrylene, k(T)) (σ/σ H+C 12 H 8) k H+C 12 H 8) (σ/2){3.27 × 10 8 T 1.71 exp(-8170/T)} (cm 3 /mol‚s), where σ is the reaction symmetry number.
Reaction pathways for polyaromatic growth in combustion environments are explored theoretically u... more Reaction pathways for polyaromatic growth in combustion environments are explored theoretically using molecular dynamic techniques. The reaction systems considered include cyclization reactions that involve H atom migration. Specifically we examine the absorption of acetylene on an aromatic ring radical. The Parallel Replica method for accelerating Molecular Dynamics calculations is used to screen reaction pathways. The information obtained through the use of Molecular Dynamics runs are then analyzed with quantum chemical methods and thermodynamic data for the key compounds.
The processes involved in soot precursor formation exhibit a wide range of timescales, spanning p... more The processes involved in soot precursor formation exhibit a wide range of timescales, spanning pico- or nanoseconds for intramolecular processes that can occur on a particle surface to milliseconds for the formation of the first soot precursors. To accurately describe the soot formation process, it is important to model the reactions happening at different timescales. The use of atomistic models
... A.VIOLI, S. YAN, EG EDDINGS*, AND AF SAROFIM ... Jet A and JP-8 are kerosene fuels used in av... more ... A.VIOLI, S. YAN, EG EDDINGS*, AND AF SAROFIM ... Jet A and JP-8 are kerosene fuels used in aviation and consist of complex mixtures of higher order hydrocarbons, including alkanes, cycloalkanes, and aromatic molecules. ...
Formation pathways for high-molecular-mass compound growth are presented, showing why reactions b... more Formation pathways for high-molecular-mass compound growth are presented, showing why reactions between aromatic moieties are needed to explain recent experimental findings. These reactions are then analyzed by using quantum mechanical density functional methods. A sequence of chemical reactions between aromatic compounds (e.g., phenyl) and compounds containing conjugated double bonds (e.g., acenaphthylene) was studied in detail. The sequence begins with the H-abstraction from acenaphthylene to produce the corresponding radical, which then furnishes higher aromatics through either a two-step radical-molecule reaction or a direct radical-radical addition to another aromatic radical. Iteration of this mechanism followed by rearrangement of the carbon framework ultimately leads to high-molecular-mass compounds. This sequence can be repeated for the formation of high-molecular-mass compounds. The distinguishing features of the proposed model lie in the chemical specificity of the routes considered. The aromatic radical attacks the double bond of five-membered-ring polycyclic aromatic hydrocarbons. This involves specific compounds that are exceptional soot precursors as they form resonantly stabilized radical intermediates, relieving part of the large strain in the five-membered rings by formation of linear aggregates.
Purpose-This paper aims to study the natural convection of a nanofluid inside a cavity which cont... more Purpose-This paper aims to study the natural convection of a nanofluid inside a cavity which contains obstacles using lattice Boltzmann method (LBM). The results have focused mainly on various parameters such as number and aspect ratio of roughness elements and different nanoparticle volume fraction. The isotherms and streamlines are presented to describe the hydrodynamics and thermal behaviors of the nanofluid flow throughout the enclosure. Design/methodology/approach-The methodology of this paper consists of mathematical model, statement of the problem, nanofluid thermophysical properties, lattice Boltzmann method, LBM for fluid flow, LBM for heat transfer, numerical strategy, boundary conditions, Nusselt (Nu) number calculation, code validation and grid independence. Findings-Natural convection heat transfers of a nanofluid inside cavities with and without rough elements have been studied. Lattice Boltzmann technique has been used as numerical approach. The results showed that at higher Rayleigh number (Ra = 10 6), there are denser streamlines near the left (source) and right wall (sink) which results in better cooling and enhances convective heat rejection to the heat sink. After a distinctive aspect ratio of rough elements (A = 0.1), change in streamline pattern which arises from increasing of aspect ratio does not have an important effect on isotherms. Results indicate that for lower Rayleigh number (Ra = 10 3), no variation in average Nu is observed with increasing in number of roughness, while for higher one (Ra = 10 6) average Nu decreases from N = 0 (smooth cavity) up to N = 4 and then remains constant (N = 6). Originality/value-Currently, no argumentative and comprehensive extraction can be concluded without fully understanding the role of different arrangement of roughness. Some geometrical parameters such as aspect ratio, number and position of rough elements have been considered. Also, the effect of nanoparticle concentration was studied at different Ra number. Briefly, using LBM, this paper aims to investigate the natural convection of a nanofluid flow on the thermal and hydrodynamics parameters in the presence of rough element with various arrangements.
Homogeneous nucleation is identified as one of the steps responsible for the formation of the cri... more Homogeneous nucleation is identified as one of the steps responsible for the formation of the critical nuclei that define the transition from gas-phase species to soot formation particles. However, the effect of molecular properties (like size, composition, and shape) on their propensity to physically bind, for a time long enough to initiate particle nucleation, has not been thoroughly investigated. In this work, we analyze the effect of oxygen content on the homodimerization propensity of different polycyclic aromatic compounds (PACs) using molecular dynamics simulations coupled with well-tempered Metadynamics enhanced sampling. We analyzed species in two mass groups, with a molecular weight ranging from 448 μ to 696 μ, and with varying oxygen content, mostly present as ethers. The reconstructed free energy profiles show that, besides mass, the presence of oxygenated groups influences the tendency of PACs to bind, with oxygenated compounds forming dimers that are less stable than the corresponding polycyclic aromatic hydrocarbons. Although this work represents only the first step towards a model for PACs dimerization that include oxygen, these results suggest that oxygenated compounds are less likely to condense onto soot particles.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
The emission standards of combustion have been steadily reduced in recent years, and a large rese... more The emission standards of combustion have been steadily reduced in recent years, and a large research effort has been focused on lowering the emissions of hydrocarbons and particulate matter. Addition of oxygenates to fuel reduces these pollutants. In this paper, we report on a detailed investigation of the growth mechanisms for gas-phase species in ethylene/air and ethylene/ethanol/air flames in order to assess the importance of various chemical mechanisms for the molecular growth of soot precursors. We employ a variety of computational techniques that include stochastic and deterministic methods to study the formation of gas-phase species and their growth into soot precursors via chemical and physical mechanisms. We have drawn the following conclusions: 1) the chemistry of oxygenated compounds (specifically the high concentrations of O 2 and OH) is critical to reproduce the experiments; 2) using free energy simulations, we found out that the tendency of molecules to form dimers is mainly affected by the molecular shape rather than the mass of the aggregate. Finally we propose a different mechanism for the growth of soot precursors based on radicalradical recombination to form molecules of high molecular masses (> 1200 u). These structures are then likely to promote physical aggregation to further the growth mechanism.
... IN HIGH-PRESSURE DIESEL-LIKE CONDITIONS ANGELA VIOLI,1 ANDREA D ANNA,1 ANTONIO D ALESSIO,1 MA... more ... IN HIGH-PRESSURE DIESEL-LIKE CONDITIONS ANGELA VIOLI,1 ANDREA D ANNA,1 ANTONIO D ALESSIO,1 MASSIMO ASTARITA2 and BIANCA MARIA VAGLIECO2 1Dipartimento di ... 12. Astarita, M., Corcione, FE, Vaglieco BM, and Val entino, G., SAE paper 99-0510. 13. ...
In this paper, we report a detailed analysis of the breakdown kinetic mechanism for methyl butano... more In this paper, we report a detailed analysis of the breakdown kinetic mechanism for methyl butanoate (MB) using theoretical approaches. Electronic structures and structure-related molecular properties of reactants, intermediates, products, and transition states were explored at the BH&HLYP/cc-pVTZ level of theory. Rate constants for the unimolecular and bimolecular reactions in the temperature range of 300-2500 K were calculated using Rice-Ramsperger-Kassel-Marcus and transition state theories, respectively. Thirteen pathways were identified leading to the formation of small compounds such as CH 3 , C 2 H 3 , CO, CO 2 , and H 2 CO. For the initial formation of MB radicals, H, CH 3 , and OH were considered as reactive radicals participating in hydrogen abstraction reactions. Kinetic simulation results for a high temperature pyrolysis environment show that MB radicals are mainly produced through hydrogen abstraction reactions by H atoms. In addition, the C(O)OCH 3) CO + CH 3 O reaction is found to be the main source of CO formation. The newly computed kinetic sub-model for MB breakdown is recommended as a core component to study the combustion of oxygenated species.
A detailed kinetic model has been developed and used to simulate aromatic growth in premixed benz... more A detailed kinetic model has been developed and used to simulate aromatic growth in premixed benzene and ethylene flames. The model considers the role of resonantly stabilized radicals in the growth of aromatic species, in addition to the hydrogen abstraction carbon addition (HACA) mechanism, which involves hydrogen abstraction to activate aromatics followed by subsequent acetylene addition. Model results show that the self-combination of resonantly stabilized radicalss in particular, the combination of cyclopentadienyl radicalssis the controlling pathway for the aromatic-ring growth. The kinetic model reproduces the experimental trends of these compounds already below the flame front and their decrease within the flame front, which has been observed experimentally but never predicted numerically. The reaction mechanism has been used to identify the different behaviors of aromatic growth in ethylene and benzene flames. Benzene formation is the rate-limiting step for aromatic growth in ethylene flames. C 6 H 6 is formed across the flame front and carbon growth continues through the formation of multi-ring aromatics in the postflame region. In benzene oxidation, the aromatic ring is already present in the main oxidation zone and it is mainly oxidized to cyclopentadienyl radicals. As a consequence, a large amount of cyclopentadienyl radicals are available for recombination reactions, leading to multi-ring formation already in the main oxidation region. The increase of the temperature at the flame front reduces their concentration due to pyrolysis and/or oxidation resulting in a peak value in the main oxidation region and a leveling-off in the post-oxidation region of the flame.
Classical molecular dynamics simulations of atomistic models of combustion-generated carbon nanop... more Classical molecular dynamics simulations of atomistic models of combustion-generated carbon nanoparticles and lipid bilayers have been performed to explore their possible structural, dynamical, and thermodynamic effects on biological membranes. The DREIDING generic force field is used for the carbonaceous nanoparticles of different morphologies, as produced from combustion sources, and the united atom model was employed for the dimyristoylphosphatidylcholine (DMPC) bilayer. It is observed that particle shape and structure have significant effects on solvation, mobility, adsorption, and permeation behavior of the particles. While combustion-generated carbon nanoparticles with an aspect ratio close to unity prefer to stay near the membrane center, precursors with other shapes mostly reside within the hydrocarbon tail region of the membrane. Carbon nanoparticles are not trapped in a local region even inside the membranes but move freely with a speed depending on their molecular weight. The adsorption of the particles on the surface of the biological membrane is comparable to thermal fluctuations because the weak segregation effect by water molecules is the main driving force to the adsorption behavior. The bigger the precursors are, the stronger they are bound to the membrane surface. The presence of combustion-generated nanoparticles inside the membrane perturbs local lipid density by pushing the neighboring lipid molecules away from the nanoparticles. This, coupled with thermal fluctuations, can induce an instantaneous membrane pore to allow water protrusion. From the umbrella sampling method, the potential of mean force for the permeation of carbona nanoparticles into the bilayer was also obtained. Surprisingly, elongated particles have a free energy barrier an order of magnitude smaller compared with more round ones. In addition, the round carbon nanoparticles showed strong hysteresis due to the local trapping of water molecules. Although the carbon soot precursors studied in this work are not the well-known carbon nanoparticles such as fullerenes or carbon nanotubes, the qualitative features of this study may be applicable to them as well.
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